1986 Volume 36 Issue 198 Pages 273-288
The Toyoha zinc-lead-silver ore deposit of vein-type is located in the Green Tuff Region of Southwest Hokkaido. In the eastern area of the Toyoha mine, ore veins are unrecognizable at the surface, but sulfide veins have been confirmed by some drilling cores at 600-1000 meters from the surface. All the rocks in the studied area composed of andesite, dacite, acidic tuff, sandstone, mudstone and conglomerate of Miocene age have been converted into various types of altered rock. The alteration intricately duplicated may be ascribed to the composite alteration of different origin. This conception was obtained by the altered mineral assemblage and the distribution, from which the diagenetic and the hydrothermal alterations were distinguished. Followings are evidences and conclusions.
(1) Alteration in the studied area is divided into four alteration zones, A, B, C and F. Moreover facies A is subdivided into subfacies A-1, A-2 and A-3. Characteristic mineral assemblage of each alteration zone is as follows. A-1: Smectite, A-2: Chlorite/smectite mixed-layer mineral-sericite/smectite mixed-layer mineral, A-3: Chlorite-sericite-quartz-calcite, B: Quartz-chlorite-albite-calcite, C: Quartz-chlorite-sericite-pyrite, F: Quartz-kaolinite-sericite.
(2) Alteration zone A is widely distributed and shows zonal distribution of subfacies A-1, A-2, A-3 downward. Alteration zones B-C and F are locally distributed around the ore veins and the junction of the Shirai and Yunosawa rivers, respectively.
(3) Alteration zones B and C are inferred to be formed by a solution associated with mineralization. Alteration zone F is a product by an acidic solution. Various hydrothermal solutions, ore solution, acidic one and so on, was very active.
(4) As alteration zone A is affected strongly by the geothermy, the terrestrial heat flow reaches 27.5 HFU, and activity of various hydrothermal solutions, it may be generated by the diagentic process at the highest geothermal gradient area.
(5) The mineral assemblage and character of clay minerals of the alteration associated with the mineralization at the studied area are similar to those at the Motoyama deposits. Nevertheless the distribution of alteration zone B is narrower and the degree of silification or pyritization is lower than that of the Motoyama deposits.
